What Is Bromine?

Bromine is a chemical element with the symbol Br and atomic number 35, belonging to the halogen group (Group 17) of the periodic table. It is one of only two elements that exist as a liquid at standard ambient temperature and pressure, appearing as a dense, reddish-brown liquid with a pungent odor. Bromine is highly reactive, forming compounds with most other elements, and is found in nature primarily as bromide salts dissolved in seawater, salt lakes, and underground brine deposits. Its chemical reactivity and the stability of organobromide compounds make bromine industrially significant across pharmaceuticals, agriculture, water treatment, and electronics manufacturing.

In electrical and electronics engineering, bromine is best known for its role in brominated flame retardants, which are added to printed circuit boards, housings, and cables to reduce fire risk. This application connects the chemistry of halogens directly to the design and safety certification of electronic systems.

Chemical Properties

Bromine (Br2, molecular weight 159.808) is a diatomic molecule in its elemental form. Its high electronegativity and the relative weakness of the Br-Br bond make it an effective electrophilic and radical halogenating agent. Organobromide compounds, in which bromine is covalently bonded to carbon, decompose more readily at flame temperatures than their organochloride counterparts, releasing hydrogen bromide (HBr), which interferes with the radical chain reactions that sustain combustion. This mechanism makes bromine more efficient per unit mass as a flame suppressant than chlorine in many applications. The thermochemical properties of bromine gas, including heat capacity and enthalpy across temperatures from 333 K to 6000 K, are documented in the NIST Chemistry WebBook, a standard reference for engineering thermodynamic calculations.

Brominated Flame Retardants

Brominated flame retardants (BFRs) are a family of organobromine compounds added to polymers, textiles, and electrical assemblies to meet fire safety standards. The five major classes include tetrabromobisphenol A (TBBPA), used primarily in printed circuit board laminates and thermoplastics; polybrominated diphenyl ethers (PBDEs), applied to plastics and foam; and hexabromocyclododecane (HBCDD), used in polystyrene insulation. In electronic equipment, BFRs are incorporated into the epoxy resin of FR-4 circuit board substrates, connector housings, and power supply enclosures. The National Institute of Environmental Health Sciences overview of flame retardants describes their mechanism of action and the health concerns associated with certain persistent BFR compounds, several of which have been restricted under the Stockholm Convention on Persistent Organic Pollutants due to their bioaccumulative properties.

Industrial Production and Regulatory Context

Bromine is produced commercially by oxidizing bromide ions in brines with chlorine gas, then stripping the elemental bromine with steam or air. Major production centers are located near the Dead Sea, in the Midwestern United States, and in China, where subsurface brine deposits contain high bromide concentrations. Global production is measured in hundreds of thousands of metric tons annually, with flame retardants and agricultural fumigants representing the largest end uses. The UN Environment Programme's assessment of flame retardants documents the international regulatory trajectory of brominated compounds, particularly the phase-out of certain PBDEs under the Stockholm Convention and the ongoing evaluation of TBBPA alternatives for electronics applications.

Applications

Bromine has applications in a wide range of fields, including:

• Printed circuit board manufacturing, where TBBPA is incorporated into FR-4 laminates to meet fire safety standards
• Water treatment, where bromine-based biocides are used in cooling towers and swimming pools
• Pharmaceutical synthesis, where organobromine intermediates appear in production pathways for sedatives, anticonvulsants, and contrast agents
• Agricultural chemistry, where methyl bromide has been used as a soil fumigant